1. Field of the Invention
The present invention relates to a radiation monitor that extracts a sample gas from an exhaust pipe or the like so as to measure the radioactive concentration of a radioactive substance in sample gas when a severe accident occurs in a nuclear reactor plant or the like.
2. Description of the Related Art
For example, as disclosed in Japanese Patent Application Laid-Open No. 2011-180061 (Patent Document 1), the measurement subject of a conventional radiation monitor is a radioactive noble gas to be exhausted from an exhaust pipe when a loss-of-primary-coolant accident occurs; the conventional radiation monitor covers the scope from the upper limit range of a normal-operation radiation monitor specified in “Guidance for Measurement of Exhausted Radioactive Substance in Power-Generation Light-Water Nuclear Reactor Plant” to the upper limit range of 3.7×106 Bq/cm3 specified in “Examination Guidance for Accidental-Condition Radiation Measurement in Power-Generation Light-Water Nuclear Reactor Plant”.
The radiation monitor includes a sampling unit that samples a sample gas, a detection unit that detects a radiation emitted from a radioactive substance in the sampled sample gas and outputs a radiation detection signal, and a measurement unit that measures a radiation, based on the radiation detection signal and remotely controls the sampling unit; the sampling unit is provided with an intake pipe that takes in a sample gas from a sampling point, a filter that collects dust floating in the sample gas, a pump that leads the sample gas, exhausted from the filter, to the detection unit and takes in the sample gas exhausted from the detection unit, an exhaust, pipe that exhausts the sample gas exhausted from the pump, a flow rate sensor that detects the flow rate of a sample gas and outputs a flow rate signal, and a pressure sensor that detects the pressure in the detection unit and outputs a pressure signal; the radiation monitor is designed paying attention mainly to the expansion of the measurement upper limit. That is to say, in order to expand the upper limit of the measurement range, a sample container in the detection unit is downsized so that the detection efficiency is decreased.
A sample gas reaches a high temperature of, for example, maximum 80° C.; however, the sampling unit is configured under the assumption that the sample gas condenses in the intake pipe while being sampled and hence no drain flows into the detection unit.
[Patent Document 1] Japanese Patent Application Laid-Open No 2011-180061
When such a radiation monitor as the conventional radiation monitor in which the detection unit is connected with the sampling unit deals with a severe accident, the accident causes the temperature of the sample gas to further increase and steam to intrude therein; therefore, there has been a problem that during the sampling, the steam condenses to form mist and the filter gets wet due to the mist, thereby obstructing the flow path; in addition, there has been a problem that due to the mist flowing into the detection unit, the surface of the sample container in the detection unit gets wet and particle-like radiation substances are accumulated thereon, thereby causing a measurement obstacle.
Furthermore, there has been a problem that because in order to expand the upper limit of the measurement range, the sample container in the detection unit is downsized so as to decrease the detection efficiency, the pressure of the sample gas becomes unstable when a drain intrudes therein.